In 2003, 39-year-old Terry Wallis uttered his first word (“mom”) in the 19 years since a car accident had left him with severe brain damage. He had spent much of the previous two decades in what neurologists call a minimally conscious state, somewhere in the gray area between coma and consciousness. In the years before his awakening, however, Wallis’s family had noticed that he was growing more alert and responsive, occasionally nodding, grunting, or even crying, until one day he spontaneously started to speak. Though Wallis still has serious impairments in memory and movement, he continues to make remarkable gains.

No one knows what spurred Wallis’s return to the waking world. But neurologist Nicholas Schiff is determined to find out. A researcher at the Weill Cornell Medical College in New York City, Schiff is one of a handful of scientists studying people like Wallis, patients who spend months or years seemingly unaware of the outside world and unable to communicate. Using new brain-imaging techniques, Schiff is hoping to better understand the complex nature of consciousness–and find ways to help treat the thousands of patients who suffer from severe consciousness disorders.

Eight months after Wallis’s first words, Schiff and his collaborators began taking snapshots of Wallis’s brain using a new method that can create detailed maps of the brain’s nerve fibers. What they found surprised them. Over the next year and a half, the researchers’ images seemed to show that Wallis’s brain had partially healed itself. But how? And what triggered the healing process?

In the last few decades, improved medical technologies have kept more people alive after brain injuries, but many of them have been left in apparently permanent states of impaired consciousness. Immediately after a severe brain injury, a patient often enters a coma–a period of unconsciousness that typically lasts days or, at most, weeks. Those who survive do not necessarily awaken; instead, they may enter a vegetative state or a minimally conscious state (MCS), which can last for years. While it’s difficult to determine how many minimally conscious patients there are in the United States (MCS was introduced as a diagnostic category only in 2002), some estimates put the number at 25,000 or more–about 10 times the number of vegetative patients. (The two conditions can be difficult even for neurologists to distinguish. Vegetative patients are defined as those totally unaware of their environment, while patients who are in a minimally conscious state may occasionally laugh or cry, reach for objects, or even respond to simple questions.)

Unlike Wallis, most patients who spend years in a minimally conscious state never wake up. Prospects for recovery diminish as time ticks by, so many doctors adopt a sort of therapeutic nihilism toward those who are persistently unconscious, assuming that their cases are hopeless, says Steven Laureys, a neurologist at the University of Liège in Belgium. MCS patients have few treatment options, and most don’t get rigorous long-term follow-up or intensive rehabilitation. Wallis, for example, had no neurologist, and not much medical history was collected during his 19-year hiatus from consciousness.